Feline immunodeficiency virus (FIV), a lentivirus of cats, is associated with feline acquired immunodeficiency syndrome (AIDS). See N. Pedersen et al., Science 235: 790 (1987). Disorders associated with FIV infection include chronic gingivitis/stomatitis, chronic upper respiratory infections, chronic enteritis, and recurrent ocular disease. See R. English et al., J. Am. Vet. Med. Assoc. 196:1116 (1990); N. Pedersen et al., Vet. Immunol. Immunopathol. 21:111 (1989); J. Yamamoto et al., J. Am. Vet. Med. Assoc. 194: 213 (1989). What is known to date of the pathogenesis of FIV infection suggests that it is a valuable animal model for other retroviral diseases, such as human immunodeficiency virus-1 (HIV-1) infection. HIV-1 and FIV belong to the lentivirus subfamily of retroviruses, have similar morphology, protein composition, and Mg.sup.2+ -dependency of their reverse transcriptases (RT). See N. Pedersen et al., Science 235:790 (1987); N. Pedersen et al., Vet. Immunol. Immunopathol. 21:111 (1989). They both display tropism for T lymphocytes and monocytes and are capable of inducing these cells to form syncytia. See D. Brunner and N. Pedersen, J. Virol. 63: 5483 (1989); M. Gardner and P. Luciw, FASEB Journal 3: 2593 (1989). HIV-1 displays a particular tropism for CD4.sup.+ lymphocytes, which leads to their gradual depletion and an inversion of the CD4.sup.+ :CD8.sup.+ ratio. See A. Dalgleish et al., Nature 312: 763 (1984). The pathogenesis of HIV-1 infection has been attributed to virus-induced reduction of CD4.sup.+ lymphocyte numbers and functions, resulting in decreased immune responsiveness and subsequent severe secondary infections. See M. McChesney and M. Oldstone, Ad. Immunol. 45: 335 (1989).
Yamamoto et al. studied the early events in the pathogenesis of FIV in kittens. See J. Yamamoto et al., Am. J. Vet. Res. 49: 1246 (1988). These kittens developed an acute infection syndrome similar to that seen in HIV-1, including low grade fever and transient generalized lymphadenopathy. More recent studies by Ackley et al., J. Virol. 64: 5652 (1990), utilized monoclonal antibodies directed against feline CD4.sup.+ and CD8.sup.+ homologues and Pan T cells to analyze lymphocyte profiles in SPF cats experimentally infected with FIV. These authors reported that a significant inversion of the CD4.sup.+ :CD8.sup.+ ratios occurred only in cats infected for 18 months or more. The inversion was associated with a decrease in absolute number of CD4.sup.+ cells and an increase in CD8.sup.+ cells.
A panel of monoclonal antibodies specific for feline T cell subsets (M. Tompkins et al., Vet. Immunol. Immunopathol. 26: 305 (1990)) has been used to analyze T cell numbers and profiles in cats naturally infected with FIV. See C. Novotney et al., AIDS 4: 1213 (1990). Similar to the observation of Ackley et al. supra, cats naturally infected with FIV have an inverted CD4.sup.+ :CD8.sup.+ ratio characterized by a selective reduction in CD4.sup.+ cells.